全文获取类型
收费全文 | 10597篇 |
免费 | 485篇 |
国内免费 | 565篇 |
出版年
2024年 | 17篇 |
2023年 | 121篇 |
2022年 | 150篇 |
2021年 | 196篇 |
2020年 | 209篇 |
2019年 | 254篇 |
2018年 | 237篇 |
2017年 | 169篇 |
2016年 | 215篇 |
2015年 | 249篇 |
2014年 | 348篇 |
2013年 | 679篇 |
2012年 | 271篇 |
2011年 | 340篇 |
2010年 | 275篇 |
2009年 | 415篇 |
2008年 | 384篇 |
2007年 | 435篇 |
2006年 | 427篇 |
2005年 | 446篇 |
2004年 | 389篇 |
2003年 | 388篇 |
2002年 | 387篇 |
2001年 | 283篇 |
2000年 | 245篇 |
1999年 | 259篇 |
1998年 | 248篇 |
1997年 | 239篇 |
1996年 | 248篇 |
1995年 | 238篇 |
1994年 | 228篇 |
1993年 | 265篇 |
1992年 | 228篇 |
1991年 | 244篇 |
1990年 | 200篇 |
1989年 | 185篇 |
1988年 | 157篇 |
1987年 | 180篇 |
1986年 | 151篇 |
1985年 | 171篇 |
1984年 | 179篇 |
1983年 | 100篇 |
1982年 | 155篇 |
1981年 | 126篇 |
1980年 | 101篇 |
1979年 | 81篇 |
1978年 | 39篇 |
1977年 | 33篇 |
1976年 | 34篇 |
1975年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 537 毫秒
901.
Following a toxic flood from a mine spill that affected over 45 km2 in Southern Spain, experiments were conducted in 1999 to test the feasibility of using crops for phytoremediation of the area, after the mechanical removal of the mud. Two cereals, barley and triticale, and two Brassicaspp., rapeseed and ethiopian mustard, were planted in three contaminated plots, 50 × 100 m each, and in a control plot outside the affected area. Soil and plant contents of As, Cd, Cu, Pb, Tl and Zn were measured and bioaccumulation coefficients (BC) were calculated at maturity. The four crops tested accumulated Cd and Zn in the above-ground biomass only in the plot on acid soil. Both species of Brassica accumulated Tl (average BC of 3.6 and 1.4 for rapeseed and mustard, respectively, in contaminated plots). None of the four crop plants accumulated As, Cu and Pb under the experimental conditions. Maximum plant uptake values from soil were 5.4 mg m–2 of As, 0.54 mg m–2 of Cd, 9.7 mg m–2 of Cu, 7.0 mg m–2 of Pb, 3.4 mg m–2 of Tl, and 260 mg m–2 of Zn. Total crop uptake gave estimates for successful phytoremediation of at least five decades, casting doubts on the feasibility of using these crops for decontamination of the area. Nevertheless, cereal grains had mineral contents below toxicity levels for livestock, therefore it might be possible to use these crops for livestock feed while reducing deep percolation and gradually removing metals from polluted soils. 相似文献
902.
The uptake of cholesterol has been characterized in leaf discs from mature leaves of sugar beet (Beta vulgaris L.). This transport system exhibited a simple saturable phase with an apparent Michaelis constant ranging from 30 to 190 M depending on the sample. When present at 10 M excess, other sterols were able to inhibit cholesterol uptake. Moreover, binding assays demonstrated the presence of high-affinity binding sites for cholesterol in purified plasma membrane vesicles. In the range 1–60 M, cholesterol uptake showed an active component evidenced by action of the protonophore carbonyl cyanide m-chlorophenylhydrazone. Energy was required as shown by the inhibition of uptake induced by respiration inhibitors (NaN3), darkness and photosynthesis inhibitors [3-(3,4-dichlorophenyl)-1,1-dimethylurea, methyl viologen]. Moreover, the process was strongly dependent on the experimental temperature. Uptake was optimal at acidic pH (4.0), sensitive to ATPase modulators, inhibited by thiol reagents (N-ethylmaleimide, p-chloromercuribenzenesulfonic acid, Mersalyl) and by the histidyl-group reagent diethyl pyrocarbonate. The addition of cholesterol did not modify H+ flux from tissues, indicating that H+-co-transport was unlikely to be involved. MgATP did not increase the uptake, arguing against involvement of an ABC cassette-type transporter. By contrast, cryptogein, a sterol carrier protein from the Oomycete Phytophtora cryptogea, greatly increased absorption. Taken together, the results reported in this work suggest that plant cells contain a specific plasma membrane transport system for sterols.Abbreviations CCCP
carbonyl cyanide m-chlorophenylhydrazone
- PCMBS
p-chloromercuribenzenesulfonic acid
- PMV
plasma membrane vesicle
- TLC
thin-layer chromatography 相似文献
903.
As part of our program to develop novel ligands for the estrogen receptor, we synthesized the series of isomeric 17alpha-(trifluoromethyl)phenylvinyl estradiols using our solid-phase organic synthesis methodology. The compounds were evaluated for their relative binding affinity (RBA) using the ERalpha-LBD and in vivo potency using the immature rat uterotrophic growth assay. The ortho-isomer had the highest RBA values, 48-223, and the highest estrogenicity in vivo. The other isomers had significantly lower affinities and were weaker agonists in the uterotrophic assay. The results suggest that introduction of substituents at the 17alpha-position of estradiol is tolerated by the ER-LBD and permit agonist responses in the intact animal, however, the effect is sensitive to the position of groups on the phenyl ring. This study demonstrates that the 17alpha-position of estradiol is a reasonable site for modification but the position and physicochemical properties of such modifications may significantly affect the affinity and efficacy of the ligand. 相似文献
904.
Previous research by the first author revealed that, relative to other modern peoples, sub-Saharan Africans exhibit the highest frequencies of ancestral (or plesiomorphic) dental traits and, thus, appear to be least derived dentally from an ancestral hominin state. This determination, in conjunction with various other lines of dental morphological evidence, was interpreted to be supportive of an African origin for modern humans. The present investigation expands upon this work by using: 1) direct observations of fossil hominin teeth, rather than data gleaned from published sources, 2) a single morphological scoring system (the Arizona State University Dental Anthropology System) with consistent trait breakpoints, and 3) data from larger and more varied modern human comparative samples. As before, a multivariate distance statistic, the mean measure of divergence, was used to assess diachronic phenetic affinities among the Plio-Pleistocene hominins and modern humans. The present study also employed principal components analysis on dental trait frequencies across samples. Both methods yielded similar results, which support the previous findings; that is, of all modern human samples, sub-Saharan Africans again exhibit the closest phenetic similarity to various African Plio-Pleistocene hominins-through their shared prevalence of morphologically complex crown and root traits. The fact that sub-Saharan Africans express these apparently plesiomorphic characters, along with additional information on their affinity to other modern populations, evident intra-population heterogeneity, and a world-wide dental cline emanating from the sub-continent, provides further evidence that is consistent with an African origin model. 相似文献
905.
Camperi SA Iannucci NB Albanesi GJ Oggero Eberhardt M Etcheverrigaray M Messeguer A Albericio F Cascone O 《Biotechnology letters》2003,25(18):1545-1548
The peptide, Ala-Pro-Ala-Arg (APAR), was selected from the screening of a tetrapeptide combinatorial synthetic library as the ligand for affinity purification of an anti-Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) monoclonal antibody (Mab) developed in mouse ascitis. The affinity chromatographic matrix obtained by attachment of APAR to agarose, having a peptide density of 0.5 mol ml–1, showed a maximum capacity of 9.1 mg Mab ml–1 and a dynamic capacity of 3.9 mg Mab ml–1. A 95% yield of electrophoretically pure anti-GM-CSF was obtained in a single step. 相似文献
906.
Biooxidation of ferrous iron by immobilized Acidithiobacillus ferrooxidans in poly(vinyl alcohol) cryogel carriers 总被引:4,自引:0,他引:4
PVA-cryogels entrapping about 109 cells of Acidithiobacillus ferrooxidans per ml of gel were prepared by freezing-thawing procedure, and the biooxidation of Fe2+ by immobilized cells was investigated in a 0.365 l packed-bed bioreactor. Fe2+ oxidation fits a plug-flow reaction model well. A maximum oxidation rate of 3.1 g Fe2+ l–1 h–1 was achieved at the dilution rate of 0.4 h–1 or higher, while no obvious precipitate was determined at this time. In addition, cell-immobilized PVA-cryogels packed in bioreactor maintained their oxidative ability for more than two months under non-sterile conditions.
Nomenclature:
C
A0 – Concentration of Fe2+ in feed stream (g l–1) C
A – Concentration of Fe2 + in outlet stream (g l– 1) D – Dilution rate of the packed-bed bioreactor (h–1) F – Volumetric flow rate of iron solution (l h–1) F
A0 – Mass flow rate of Fe2+ in the feed stream (g h–1) K – Kinetic constant (l l–1 h–1) r
A – Oxidation rate of Fe2+ (g l–1 h–1) V – Volume of packed-bed bioreactor (l) X
A – Conversion ratio of Fe2+ (%) 相似文献
907.
Balan S Murphy J Galaev I Kumar A Fox GE Mattiasson B Willson RC 《Biotechnology letters》2003,25(13):1111-1116
The affinity of metal chelates for amino acids, such as histidine, is widely used in purifying proteins, most notably through six-histidine `tails'. We have found that metal affinity interactions can also be applied to separation of single-stranded nucleic acids through interactions involving exposed purines. Here we describe a metal affinity precipitation method to resolve RNA from linear and plasmid DNA. A copper-charged copolymer of N-isopropyl acrylamide (NIPAM) and vinyl imidazole (VI) is used to purify plasmid from an alkaline lysate of E. coli. The NIPAM units confer reversible solubility on the copolymer while the imidazole chelates metal ions in a manner accessible to interaction with soluble ligands. RNA was separated from the plasmid by precipitation along with the polymer in the presence of 800 mM NaCl. Bound RNA could be recovered by elution with imidazole and separated from copolymer by a second precipitation step. RNA binding showed a strong dependence on temperature and on the type of buffer used. 相似文献
908.
This study was designed to measure the effect of iron supplementation on antioxidant status in iron-deficient anemia, including
the time for hemoglobin normalization and at the time of filling of iron body stores. The extent of plasma lipid peroxidation
was evaluated by measuring the levels of malondialdehyde and glutathione peroxidase (GSH-Px), and the activities of superoxide
dismutase (SOD) and catalase in 63 patients with iron-deficiency anemia before and after 6 wk of iron supplementation and
at the time when body iron stores are saturated. After 6 wk of iron supplementation, a significant decrease of oxidative stress
was observed in the treated subjects relative to controls (p<0.05). No significant differences existed between treated patients at 6 wk and at the end of the study. The erythrocyte levels
of catalase, SOD, and GSH-Px were significantly lower in treated patients relative to controls (p<0.05). These levels increased after 6 wk of supplementation (p<0.05) and showed no significant differences with those at the end of the study. 相似文献
909.
Zinc ions are essential, but at elevated concentrations, they also have toxic effects on mammalian cells. Zinc plays a crucial
role in cell proliferation and differentiation and it even protects cells against apoptosis caused by various reagents. On
the other hand, zinc at high concentrations causes cell death that was characterized as apoptotic by internucleosomal DNA
fragmentation, formation of apoptotic bodies, and breakdown of the mitochondrial membrane potential. In the present work,
a clone of rat C6 glioma cells that was resistant to toxic effects of ZnCl2 up to 250 μM was employed to study the effect of the ionophore A23187 on zinc-induced apoptosis. Neither 150 μM Zn2+ nor 100 nM A23187 alone caused apoptosis as measured by internucleosomal DNA fragmentation. However, combined exposure of C6 cells to
100 nM A23187 and 150 μM Zn2+ for 48 h was effective in inducing apoptosis. Because the so-called calcium ionophore A23187 is not specific for Ca2+ ions but also transports Zn2+ with high selectivity over Ca2+, we investigated whether this substance promoted the uptake of Zn2+ ions into C6 cells. Employing the zinc-specific fluorescence probe Zinquin, we observed that the very low concentration of
1.9 nM A23187 significantly and rapidly raised the intracellular mobile Zn2+ content. Analysis by atomic absorption spectroscopy revealed that incubation with 1.9 nM A23187 caused a doubling of the total intracellular zinc level within 60 min. We conclude that the apoptosis evoked by the
combined action of Zn2+ and A23187 was the result of enhanced Zn2+ influx evoked by the ionophore, resulting in higher intracellular zinc levels. 相似文献
910.
Effects of protein deficiency on liver trace elements and antioxidant activity in carbon tetrachloride-induced liver cirrhosis 总被引:1,自引:0,他引:1
González-Reimers E López-Lirola A Olivera RM Santolaria-Fernández F Galindo-Martín L Abreu-González P Sánchez-Sanchez JJ Martínez-Riera A 《Biological trace element research》2003,93(1-3):127-139
In liver cirrhosis, liver tissue becomes progressively substituted by fibrosis, ultimately leading to architectural distortion,
liver circulatory changes, and liver failure. Some data support the hypothesis that protein undernutrition may play a role
in the development and progression of nonalcoholic liver cirrhosis and that this progression is at least partially mediated
by changes in glutathione peroxidase (GPX), superoxide dismutase (SOD), and other antioxidative systems, leading to an increase
in lipid peroxidation. We analyzed the effects of protein deficiency on liver Cu, Fe, Zn, Mn, and Se in carbon tetrachloride
(CCl4)-induced liver cirrhosis, the relation of protein undernutrition and these trace elements with the activity of some hepatic
antioxidative enzymatic mechanisms, and the relation of all of them with morphological and biochemical changes in 40 male
adult Sprague-Dawley rats divided in four groups. Liver cirrhosis was induced by intraperitoneal injection of CCl4 to 10 rats fed a 2% protein diet and another 10 fed a 18% protein control diet; two further groups included rats without
cirrhosis fed the 2% protein and the 18% protein diets. The study period lasted 6 wk. GPX, SOD, and lipid peroxidation products
as well as Zn, Cu, Mn, Se, and Fe were determined in liver samples. We found that liver GPX and Se were reduced in the cirrhotic
animals, especially in the low-protein-fed ones, protein deficiency, but not cirrhosis, exerting the main effects. A close
correlation was found between liver GPX and serum albumin and weight loss and an inverse one among GPX and hepatocyte ballooning,
liver fibrosis, and fat, histomorphometrically determined. These results suggest a pathogenetic role of decreased GPX in the
progression of liver disease, which may become enhanced by concomitant protein undernutrition. In addition to iron, the levels
of which were increased in the malnourished rats, no differences were found regarding the other trace elements, SOD activity,
and lipid peroxidation products. 相似文献